[en] We present the results of x-ray and ultraviolet photoelectron spectroscopy of NpPd₃ and PuPd₃. The spectra indicate that for both compounds, the 5f electrons are well localized on the actinide sites. Comparison with bulk measurements indicates that for NpPd₃ the electrons have a valence of Np³⁺ and thus a ground state 5f⁴ with a Hund's rules ⁵I₄ configuration. Similarly for PuPd₃, we find a Pu³⁺ valence, 5f⁵ ground state and a Hund's rules ⁶H_5/2 configuration

[en] We have used x-ray resonant scattering techniques to make a comprehensive study of the quadrupolar order in UPd₃. New measurements reveal that the order parameter describing the in-phase stacking along the c-axis of the quadrupole moments of the uranium ions on the quasi-cubic sites is primarily Q_xy in the two lowest temperature phases. These results are discussed within the context of a three-level crystal field model for the uranium 5f² electrons, the allowed quadrupolar operators, and the constraints deduced from the analysis of heat capacity measurements. We have now deduced, and summarize, the order parameters for each of the four antiferroquadrupolar phases of UPd₃

[en] This report presents azimuthal dependent and polarization dependent x-ray resonant magnetic scattering at the Ir L₃ edge for the bilayered iridate compound Sr₃Ir₂O₇. The two magnetic wave vectors, k₁=( 1/2 , 1/2 ,0) and k₂=( 1/2 ,- 1/2 ,0), result in domains of two symmetry-related G-type antiferromagnetic structures, denoted A and B, respectively. These domains are approximately 0.02 mm² and are independent of the thermal history. An understanding of this key aspect of the magnetism is necessary for an overall picture of the magnetic behaviour in this compound. The azimuthal and polarization dependence of the magnetic reflections, relating to both magnetic wavevectors, show that the Ir magnetic moments in the bilayer compound are oriented along the c axis. This contrasts with single layer Sr₂IrO₄ where the moments are confined to the ab plane. (fast track communication)

[en] X-ray magnetic critical scattering measurements and specific heat measurements were performed on the perovskite iridate . We find that the magnetic interactions close to the Néel temperature are three-dimensional. This contrasts with previous studies which suggest two-dimensional behaviour like Sr₂IrO₄. Violation of the Harris criterion () means that weak disorder becomes relevant. This leads a rounding of the antiferromagnetic phase transition at , and modifies the critical exponents relative to the clean system. Specifically, we determine that the critical behaviour of is representative of the diluted 3D Ising universality class. (paper)

[en] Soft x-ray resonant scattering (XRS) has been used to observe directly, for the first time, the ordering of localized electronic states on both the Mn and the Tb sites in multiferroic TbMnO₃. Large resonant enhancements of the x-ray scattering cross-section were observed when the incident photon energy was tuned to either the Mn L or Tb M edges which provide information on the Mn 3d and Tb 4f electronic states, respectively. The temperature dependence of the XRS signal establishes, in a model independent way, that in the high-temperature phase (28 K≤T≤42 K) the Mn 3d sublattice displays long-range order. The Tb 4f sublattices are found to order only on entering the combined ferroelectric/magnetic state below 28 K. Our results are discussed with respect to recent hard XRS experiments (sensitive to spatially extended orbitals) and neutron scattering. (fast track communication)

[en] Following the controversy between two previous publications (Lorenzo et al 2008 Phys. Rev. Lett. 101 226401 and Garcia et al 2009 Phys. Rev. Lett. 102 176405), we report on the influence of mechanical polishing, and subsequent sample storage, on the electronic order at the Verwey transition of highly pure magnetite, Fe₃O₄, by resonant x-ray scattering. Contrary to expectations, mechanically polishing the surface induces an inhomogeneous micron deep dead layer, probably of powdered Fe₃O₄. In addition, we have found that polishing the sample immediately before the experiment influences and favors the appearance of long range order electronic correlations, whereas samples polished well in advance have their electronic order quenched. Conversely, lattice distortions associated with the Verwey transition appear less affected by the surface state. We conclude that mechanical polishing induces stresses at the surface that may propagate into the core of the single crystal sample. These strains relax with time, which affects the different order parameters, as measured by x-ray resonant diffraction.

[en] Sr₂IrO₄ is a prototype of the class of Mott insulators in the strong spin–orbit interaction (SOI) limit described by a J_eff = 1/2 ground state. In Sr₂IrO₄, the strong SOI is predicted to manifest itself in the locking of the canting of the magnetic moments to the correlated rotation by 11.8(1)° of the oxygen octahedra that characterizes its distorted layered perovskite structure. Using x-ray resonant scattering at the Ir L₃ edge we have measured accurately the intensities of Bragg peaks arising from different components of the magnetic structure. From a careful comparison of integrated intensities of peaks due to basal-plane antiferromagnetism, with those due to b-axis ferromagnetism, we deduce a canting of the magnetic moments of 12.2(8)°. We thus confirm that in Sr₂IrO₄ the magnetic moments rigidly follow the rotation of the oxygen octahedra, indicating that, even in the presence of significant non-cubic structural distortions, it is a close realization of the J_eff = 1/2 state. (fast track communication)

[en] X-ray diffraction, specific heat, magnetic susceptibility and inelastic x-ray scattering measurements on the transurarium oxypnictides NpFeAsO and NpFeAsO_0_._8_5F_0_._1_5 are presented. No superconductivity down to 2 K was observed upon fluorine doping, contrary to the structurally analogous rare-earth pnictides. No modification of the phonon density of states was observed upon doping with fluorine. We discuss our results in light of the latest experimental and theoretical studies on the role of phonons in the superconducting pnictide compounds. (paper)